Gas discharge ultraviolet lasers used as light sources for integrated circuit lithography typically are line narrowed. A preferred line narrowing prior art technique is to use a diffraction grating based line narrowing unit along with an output coupler to form the laser resonant cavity. The gain medium within this cavity is produced by electrical discharges into a circulating laser gas such as krypton, fluorine and neon (for a KrF laser); argon, fluorine and neon (for an ArF laser); or fluorine and helium and/or neon (for an F2 laser).
A sketch of such a prior art system is shown in FIG. 1 which is extracted from Japan Patent No. 2,696,285. The system shown includes output coupler (or front mirror) 4, laser chamber 3, chamber windows 11, and a grating based line narrowing unit 7. The line narrowing unit 7 is typically provided on a lithography laser system as an easily replaceable unit and is sometimes called a xe2x80x9cline narrowing packagexe2x80x9d or xe2x80x9cLNPxe2x80x9d for short. This unit includes two beam expanding prisms 27 and 29 and a grating 16 disposed in a Litrow configuration so that diffracted beam propagates right back towards the incoming beam. The output of these excimer lasers are typically rectangular with the long dimension of for example 20 mm in the vertical direction and a short dimension of for example 3 mm in the horizontal direction. Therefore, in prior art designs, the beam is typically expanded in the horizontal direction so that the FIG. 1 drawing would represent a top view.
Another prior art excimer laser system utilizing a diffraction grating for spectrum line selection is shown in FIG. 2. The cavity of the laser is created by an output coupler 4 and a grating 16, which works as a reflector and a spectral selective element. Output coupler 4 reflects a portion of the light back to the laser and transmits the other portion 6 which is the output of the laser. Prisms 8, 10 and 12 form a beam expander, which expands the beam in the horizontal direction before it illuminates the grating. A mirror 14 is used to steer the beam as it propagates towards the grating, thus controlling the horizontal angle of incidence. The laser central wavelength is normally changed (tuned) by turning very slightly that mirror 14. A gain generation is created in chamber 3.
Diffraction grating 16 provides the wavelength selection by reflecting light with different wavelengths at different angles. Because of that only those light rays which are reflected back into the laser will be amplified by the laser gain media, while all other light with different wavelengths will be lost.
The diffraction grating in this prior art laser works in a Littrow configuration, when it reflects light back into the laser. For this configuration, the incident angle xcex1 and the wavelength xcex are related through the formula:
2dnsinxcex1=mxcexxe2x80x83xe2x80x83(1) 
where xcex1 is the incidence angle on the grating, m is the diffraction order, n is refractive index of gas, and d is the period of the grating.
Because microlithography exposure lenses are very sensitive to chromatic aberration of the light source, it is required that the laser produce light with very narrow spectrum line width. For example, state of the art excimer lasers are now producing spectral line widths on the order of 0.5 pm as measured at full width at half maximum values and with 95% oldie light energy concentrated in the range of about 1.5 pm. New generations of microlithography exposure tools will require even tighter spectral requirements. In addition, it is very important that the laser central wavelength be maintained to very high accuracy as well. In practice it is required that the central wavelength is maintained to better than 0.05-0.1 pm stability.
A need exists for greater narrowing of the laser beam.
The present invention provides for a grating based line narrowing unit with bi-directional beam expansion for line narrowing lasers. In a preferred embodiment a beam from the chamber of the laser is expanded in the horizontal direction with a three-prism beam expander and is expanded in the vertical direction with a single prism. A narrow band of wavelengths in the expanded beam is reflected from a grating in a Littrow configuration back via the two beam expanders into the laser chamber for amplification.